Field of the Invention
The invention relates to a method and a device for the definition and/or adjustment, specifically the regulation and/or control of the dead time between the opening of a first switching element and the closing of a second switching element in a switching power supply unit with active freewheeling.
Many switching power supply units for converting a DC input voltage into a supply voltage incorporate active freewheeling, wherein a first switching element is serially-connected to a second switching element, and the second switching element assumes the active freewheeling function. The DC input voltage is supplied to the switching controller. In parallel with the second switching element, an inductance is serially-connected to a capacitance. An output voltage or measuring voltage is tapped from the second switching element. The supply voltage for the supply of a consumer is tapped from the capacitance. The first and second switching elements are periodically opened and closed, whereby at least one of the switching elements is open at any time. The ratio of the closing time of the first switching element to the total duration of the closing time and the subsequent opening time of the first switching element is described as the pulse duty factor. Using the pulse duty factor, for a given DC input voltage and a given electrical consumer which is parallel-connected to the capacitance, a required supply voltage can be set. Various forms of this basic switching power supply unit with active freewheeling, such as step-down converters or buck converters, will be known to a person skilled in the art.
The time interval from the opening of the first switching element to the closing of the second switching element, during which both switching elements are thus open, is described as “dead time”.
Switching elements can be configured as transistors, for example as metal oxide field effect transistors (MOSFETs). By design, MOSFETs of this type cannot execute abrupt, i.e. infinitesimally brief switching operations, but require a certain time to open and close, dictated by their production technology and geometry, ranging from a few tenths of a nanosecond up to a few nanoseconds. Moreover, technology dictates that MOSFETs incorporate parasitic diodes between a drain terminal and a source terminal. A parasitic diode of this type on the second switching element acts in parallel with the series circuit formed by the inductance and the capacitance, from which the supply voltage is tapped.
By its manufacture, on the grounds of the filamentary, or at least elongated metal connections between the components, a switching controller of this type constitutes a stray inductance. As a result of the recovery behavior of the parasitic diode on the second switching element, this stray inductance, depending upon the response time of the switching elements, the DC input voltage and the electrical consumer which is parallel-connected to the capacitance, in the event of an excessive dead time between switching operations, resonance phenomena can occur, such that the output voltage or measuring voltage, and thus also the supply voltage, is subject to the superimposition of voltage spikes. These voltage spikes can be observed as a short-term overvoltage. The short-term overvoltage, and a resulting short-term overcurrent, result in an undesirably high electrical emission.
Moreover, in the event of an excessively short dead time, the switch-out phase of the first switching element and the switch-in phase of the second switching element can overlap. An overlap of this type will also result initially in an overvoltage on the measuring output of the switching controller. A further reduction of the dead time can result in a high short-circuit current in both switching elements, potentially leading to the destruction of the switching elements.